Compared to a frequency (for example, 800 MHz, 2000 MHz, or the like) used in a wireless terminal called a third generation or a fourth generation, a millimeter-wave band such as 24.25 GHz, 28 GHz, or 39 GHz has been determined to be allocated to a frequency to be used in the next generation (a fifth generation), and accordingly, a technique for measuring a directional characteristic of an antenna of a wireless terminal that uses the millimeter-wave band is necessary.
Generally, measurement of a directional characteristic of an antenna refers to measurement of a field intensity or the magnitude of radiation power (hereinafter, referred to as a radio wave intensity) in a far field, in which a distance condition of the far field is measured in a state where a distance R between transmission and reception antennas satisfies the following condition.R≥2D2/λ,
Here, λ represents a free space wavelength (m) of a radio wave to be used, and D represents a larger diameter (m) among maximum diameters of openings of the transmission and reception antennas.
In a case where D is set to four times the wavelength λ, R≥2D2/λ=λ(16λ2)/λ=32λ, and in a case where the frequency of a radio wave is set to 30 GHz, λ=10 mm, and the distance R necessary for far field measurement becomes equal to or longer than 32 cm.
In the case of real far field measurement, under an environment such as a radio free-field room, in a state where transmission and reception antennas are spaced from each other by the distance R and a radio wave is transmitted or received to be able to measure the radio wave intensity, one antenna that is a measurement target fixed to a rotatable antenna installation unit is rotated, for example, in a horizontal direction and a vertical direction centering around a phase center thereof at a predetermined angle step, and the radio wave intensity is measured at each rotation angle, and thus, a directional characteristic of the one antenna is calculated in all directions.
A phase center of an antenna refers to a point that is virtually considered as a radio wave concentration point in emission and incidence of a radio wave, and its position depends on the type of the antenna. For example, in the case of a dipole antenna, a point in the vicinity of a power supply point becomes a phase center. In the case of a horn antenna, a phase center is positioned on a slightly inner side of a horn on a central line of a horn opening portion. In an ideal antenna, a phase center is determined as a single point, but in a real antenna, such points are scattered due to various reasons. Here, an average position thereof is defined as an antenna phase center, and it is considered that the position scattering of the phase center of the antenna itself is enough small to be ignored compared with other errors of a measurement system. Further, in the following description, “the position of an antenna” does not represent a physical position antenna having a stereoscopic or planar extension, but represents a phase center that serves as an electric function of an antenna, unless otherwise mentioned.
The above-mentioned distance condition of the far field measurement is disclosed in the following Non-Patent Documents 1 and 2, for example.